Neutron stars form in supernova explosions that mark the death of stars several times more massive than the Sun. Although they contain only a little more mass than our Sun, this mass is crushed into a sphere only 10-20 km in radius. The resulting gravity is so strong that neutrons are squeezed out of atomic nuclei. Nuclear physicists do not know exactly what happens under these conditions: neutrons may even disintegrate into their constituent quarks, forming all kinds of exotic particles. This makes neutron stars a fantastic laboratory for extreme physics - they have crushing gravity, unknown nuclear physics, and super-strong magnetic fields (a staggering ten thousand billion times stronger than the Earth's magnetic field). In studying neutron stars we are testing the laws of physics in environments far beyond those that we can generate here on Earth.

One way to figure out what an object is made of is to study its seismic vibrations. This is the approach that is used by geologists and geophysicists, for example, to figure out the interior structure of the Earth. Studying the effect of starquakes on neutron stars if of course rather harder (since we can't go and place a seismometer on the star's surface!) but the telltale signs of starquakes can be seen by studying the electromagnetic radiation emitted by the star. Over the last few years I have been closely involved in studying starquakes on a particular class of neutron stars called magnetars, which have very strong magnetic fields. These strong fields get tangled up and reconnect, launching strong gamma-ray flares, in a process similar to that which launches solar flares. As the magnetic field lines twist and snap they can set the star vibrating. By studying the frequencies of the vibrations excited by the flares, we have been trying to figure out the properties of the star's interior. At present we are particularly interested in what excites the bursts, how to detect vibrations after small bursts, and the associated emission processes.

Another process that can excite oscillations on neutron stars is a Type I X-ray burst - a violent thermonuclear explosion that occurs on the surface layers of a neutron star as they gobble up matter from a companion star. The explosions shake up the atmosphere and ocean of the neutron star. Imagine detonating a bomb at the bottom of a pond, and trying to figure out what happened by studying the waves that make it to the surface! The patterns that form can also tell us much about the star's mass and radius, which depend on their unknown interior composition.

A neutron star's strong gravity also gives us one additional and very novel way of studying their vibrations - using gravitational waves. A newborn neutron star, for example, cannot be studied easily using traditional electromagnetic astronomy - since the radiation cannot penetrate the thick clouds of debris from the supernova explosion. However the hot newborn star should be set into vibration by the explosion, and as it wobbles its gravitational field will vary. This information is communicated to the rest of the Universe as gravitational waves. These pass through the debris cloud unimpeded. Gravitational waves are phenomenally hard to detect, but there is huge experimental progress in this area at the moment, and indeed the very first direct detection (albeit of black holes rather than neutron stars) has just been announced!